The invention relates to a constant velocity universal ball joint comprising an outer joint part having a first axis and comprising an inner aperture in which there are provided substantially longitudinally extending first ball tracks, an inner joint part having a second axis and forming a ball hub positioned in the inner aperture of the outer joint part and being provided with substantially longitudinally extending second ball tracks, torque transmitting balls guided in first and second ball tracks associated with one another in pairs, and an annular ball cage which is arranged between the outer joint part and inner joint part and comprises circumferentially distributed ball windows in which the individual balls are held in a common plane and guided on to the angle-bisecting plane between the first axis and the second axis.
It is known that the torque-related pressure conditions in the ball tracks and thus the service life of joints of the above type can be influenced by the cross-sectional shape of the ball tracks. In principle, in a cross-section through the joint, the local radius of curvature of the ball tracks in the load-transmitting points of contact with the balls is greater than the ball radius. Hereafter, the ratio of the two radii is referred to as conformity factor. By definition, it is always greater than 1 and its numerical value increases with a decreasing osculation, i.e. with a decrease in equality of shape and in the conformity of the two curvatures in the lexical meaning.
The publications listed below describe different shapes of the cross-sections of the outer and inner ball tracks of the joints of the species mentioned.
DE-GM 1 831 827
DE-PS 1 126 199
DE-PS 1 169 727
DE-OS 1 675 240
DE-PS 2 433 349.
Typical track cross-sections are circular-arch-shaped cross-sections wherein the ball of a joint in the unloaded condition is in contact with the track base in one point; cross-sections in the form of a portion of an ellipse wherein the long axis of the ellipse forms the central axis of the track cross-section and wherein the ball of a joint in the unloaded condition establishes contact in two points at a distance from the track base; and the gothic or pointed-arch-shaped cross-section wherein the centers of curvature of the two flanks of the pointed arch are offset relative to the central axis of the track cross section and wherein the ball, with the joint being in the unloaded condition, establishes contact in two points at a distance from the track base. Furthermore, there are known track cross-sections with a flattened track base which, when the joint is in the unloaded condition, generate a three-point contact of the ball in the ball track.
Joints with a circular-arch-shaped track cross-section and one-point contact with the ball are advantageous in that a pressure load can be applied along the entire height of a track flank, i.e. from the track base right up to the track edge. As a resultxe2x80x94provided the track conformity is suitably dimensionedxe2x80x94it is possible to achieve the lowest pressure values, i.e. the highest possible service life values. However, this is the case only if the ball is mounted in the ball track in as play-free a way as possible (small radial play relative to the outer part and hub). If this cannot be achieved or if, during operation, the running-in wear (mainly in the cage windows) leads to an increase in play, there is a risk of the occurrence of edge bearing, which can lead to a considerable reduction in service life values. A further disadvantage of the track with a circular-arch-shaped cross-section is the considerable effect which a radial ball/track play has on the rotational play of the joint. Joints with an elliptical track cross-section and a two-point contact with the ball are much more tolerant in respect of rotational play than the previously mentioned joints, although the service life values of said track/ball configuration are regarded as insufficient because only a very small portion of the track is subjected to pressure, so that there therefore have to be generated higher pressure values than in the case of the previously mentioned joints.
In calculations, joints with a circular-arch-shaped track cross-section with one-point contact have better service life values than joints with an elliotical or pointed-arch-shaped track cross-section with two-point contact, one reason being that, in the joints mentioned first, the Hertzian pressure ellipse in the track/ball point of contact is greater than in the joints mentioned in second place. The Hertzian stress values of the track/ball configuration mentioned first are thus lower than those of the track/ball configuration mentioned in second place. However, one disadvantage of joints with a circular-arch-shaped track cross-section consists in that, when high torque values are transmitted, the Hertzian pressure ellipse reaches the track edge, so that excessive loads (stress peaks) can lead to plastic deformation and/or definite damage to the track edge. In particular, this occurs in the outer joint part where the track depth is usually much smaller than in the inner joint part, as a result of which the pressure ellipse reaches the track edges earlier in the outer joint part than in the inner joint part.
Using track cross-section, track, track base, track edge and ball in the singular in the above paragraphs is simpler from the point of view of language and does not contradict the fact that there is provided a plurality of same in each joint.
It is the object of the present invention to provide bail joints with a track shape wherein, even when subjected to high torque values, the tracks edges are kept largely free from load peaks. A general solution in this respect consists in that at least the ball tracks of one of the joint partsxe2x80x94outer joint part or inner joint partxe2x80x94comprises a track cross-section which remains uniform along the length of same, whose second derivative rises continuously and monotonically from the track base, whose radius of curvature is not constant and which, when the joint is torque-free, permits a contact between the respective ball and the track base within a range of xe2x89xa65xc2x0 measured from the track base. In another aspect, the outer joint part or inner joint part ball tracks have a track cross-section which is uniform along a length of the ball track and whose radius of curvature axes continuously and monotonically from the track base. A special solution consists in that said ball tracks have a track cross-sections which permits contact between the respective ball and the ball track in the track base, and especially that said ball tracks comprise an elliptical cross-section with a greater axis which is positioned radially relative to the axis of the respective joint part, which elliptical cross-section permits contact between the respective ball and the ball track in the vertex of the ellipse, which vertex is positioned in the track base.
The track cross-section proposed in accordance with the invention with an approximate or even ideal track base contact is able, largely, to combine the advantages of a high pressure load even on the track base with a reduced sensitivity to play. Furthermore, by selecting a specific track shape (i.e. especially the dimensions of an ellipse) track edge bearing can be avoided, even if the balls are subject to large amounts of play in the tracks. A track cross-section in accordance with the invention is compatible with hard machining processes presently used (grinding with a disc, grinding with a pin, milling with a disc, milling with a pin). Each machine which, at present, is used for producing ball tracks should be suitable for the cross-sectional track shape proposed. For measuring the finished tracks, it is common practice in mass production to make the contact angle visible and measure the base play by touching the tracks with a test ball (diameter corresponds to the nominal ball diameter). This measuring method can also be applied to the inventive track/ball configuration, the only difference being that the test ball to be used for this purpose should have a slightly greater diameter than the ball subsequently used in the joint.
A play-free assembly should be given preference. This car be achieved by observing correspondingly close production tolerances or by classifying the components.
The conformity factor of the ball tracks with an elliptical cross-section should have standard values and, in particular, it should be below 1.02 in the region of the track base. The ellipse ratio of the elliptical tracks should range between 1.004 and 1.02. If the tracks in only one of the joint parts, either in the outer joint part or in the inner joint partxe2x80x94are designed in accordance with the invention, the outer joint part should be given preference.
The effect of the invention is that, with low torque values, the contact angle between the ball and the inventive ball track initially increases in the usual way, i.e. the contact point wanders from the track base up the track flank, with the counter forces in the respective other ball track being formed by a contact with a track flank. The ball is centered between diagonally opposed flanks of the outer ball tracks and of the inner ball track. However, due to the fact that the conformity factor of the inventive ball track rises from the track base to the track edge, a further increase in torque only negligibly results in a change in the contact angle; it is essentially only the depth of the pressure distributing ellipse under the Hertzian pressure ellipse which increases. On the other hand, a further movement of the Hertzian pressure ellipse towards the track edges is largely prevented and in consequence and in particular, a further rise of the ball at the flanks of the inventive ball track is also largely prevented. The effective contact angle at the inventive ball track is thus quasi forced on to the other ball tracks in the form of a contact angle of the same size.